1. Field of the Invention
The present invention relates to a velocity meter capable of measuring velocity of a moving object or a flowing fluid in a non-contact manner. More particularly, the present invention is concerned with a Doppler velocity meter which applies a laser beam to a moving object or fluid and measures the velocity by sensing a shift in the frequency of the laser beam.
2. Description of the Related Art
Hitherto, a laser Doppler velocity meter has been known and used which is capable of measuring the moving velocity of an object or a fluid in a non-contact manner and with a high degree of precision. Briefly, a laser Doppler velocity meter is a device which measures the velocity of a moving object or a flowing fluid by making use of a phenomenon called Doppler effect, i.e., a phenomenon in which the frequency of a laser beam scattered by the object or fluid is shifted from the frequency of the laser beam applied to the object or the fluid in an amount which is proportional to the velocity of the moving object or the fluid.
Referring to FIG. 1, a known laser Doppler velocity meter has, as its major components, a laser 1, a collimator lens 2, a parallel light beam 3, a beam splitter 4, reflective mirrors 6, 6', a condenser lens 8 and a photo-detector 9. Numeral 7 denotes an object or a fluid moving at a velocity V in the direction of an arrow.
A laser beam emitted from a laser 1 is changed by a collimator lens 2 into a collimated light beam 3 which is then split into two light beams 5 and 5' by a beam splitter 4. These light beams 5, 5' are reflected by the reflective mirrors 5, 5' and are applied to the object or fluid 7 moving at the velocity V at an incident angle represented by .theta.. The lights reflected and scattered by the object or the fluid 7 are detected by the photo-detector 9 through the condenser lens 8. The frequencies of the scattered lights are respectively Doppler-shifted in amounts of +.DELTA.f and -.DELTA.f which are proportional to the moving velocity V. Representing the wavelength of the laser beam by .lambda., the amount .DELTA.f of frequency shift is expressed by the following formula (1). EQU .DELTA.f=V sin .theta./.lambda. (1)
The scattered lights the frequencies of which have been shifted in amounts of +.DELTA.f and -.DELTA.f, respectively, interfere with each other so as to cause a periodic change of brightness on the light-receiving surface of the photo-detector 9. The frequency F of the above-mentioned periodic change in the brightness is given by the following formula (2). EQU F=2.DELTA.f=2V sin .theta./.lambda. (2)
It is therefore possible to determine the velocity of he moving object or the flowing fluid 7 in accordance with formula (2) above, by measuring the frequency output from the photo-detector 9.
In order to obtain a compact laser Doppler velocity meter, it is preferred to use a semiconductor laser such as a laser diode rather than a gas laser such as He-Ne laser as the light source. In general, however, the emission wavelength of semiconductor laser is not so stable as that of a gas laser, and undesirably changes depending on temperature. Since the Doppler frequency F depends on the wavelength .lambda. of the laser beam as will be seen from formula (2) above, the precision of detection of the moving velocity of the object or fluid is undesirably impaired by a fluctuation in the wavelength .lambda. of the laser beam.
FIG. 2 is a graph showing temperature-dependency of emission wavelength of a commercially-available laser diode, extracted from the Mitsubishi Semiconductor Data Book, 1987, Edition for Photo-semiconductor Devices). In this Figure, linear portions of the curve are caused mainly change in the refractive index of the active layer of the laser diode due to a change in temperature. The rate of change in the wavelength is about 0.05 to 0.06 nm/.degree.C. On the other hand, non-linear change in the emission wavelength is attributed to a phenomenon known as "longitudinal mode hopping", in which the wavelength varies at a large rate of 0.2 to 0.3 nm/.degree.C.
Thus, the emission wavelength of the laser diode is quite unstable. When this type of laser is used as the light source of a laser Doppler velocity meter, it is necessary to mount, together with the laser diode, a temperature control unit which is composed of, for example, a heater, a heat radiator and a temperature sensor. The use of such a temperature control unit undesirably increases the size of the velocity meter and raises the cost uneconomically.
It is also to be noted that the longitudinal mode hopping is caused also by a reason other than a temperature change. It is impossible to completely suppress a fluctuation in the emission wavelength of a laser diode even when a temperature control unit is used together with the laser diode.